The present invention generally relates to an implantable cardiac stimulation device. The present invention more particularly relates to such a device and a method utilizing spectral analysis of electrograms to administer arrhythmia therapy.
Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to a heart to cause a heart, which would normally or otherwise beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functionalities of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation therapy when it is required.
One of the primary limitations of implantable cardiac devices is their size. Size limits body location of the implant and the number of patients capable of receiving the device. For these reasons, size has always been and continues to be a factor in distinguishing one device from another.
The size of an implantable cardiac device, especially those having defibrillation capability, is driven by two primary factors, the power or battery source and capacitors. Lower defibrillation thresholds will permit smaller sized batteries and capacitors in such devices. If the number of defibrillation shocks is kept constant, the total amount of required stored energy will be reduced if the amount of energy required for each shock is reduced. This permits a smaller battery to be used.
Capacitor size is influenced by voltage rating and capacitance value. Lower defibrillation thresholds permits employment of capacitors having lower voltage ratings and less capacitance and hence, smaller size. Reduced voltage rating also has other side benefits in circuit and component design.
The goal in delivering a defibrillation shock to a heart is to depolarize a sufficient number of myocardial cells to break the fibrillation wave cycles. Hence, if the defibrillation shock could be delivered at a time when the maximum number of myocardial cells are already intrinsically depolarized, a fewest number of cells are left remaining to be depolarized by the defibrillation shock to effect defibrillation. At this point in time, the defibrillator threshold will be at a minimum. Hence, it would be advantageous to be able to discern when the point of minimum threshold is occurring and then make practical use of it to lower defibrillation thresholds with the end result of being able to reduce the size of the battery and capacitors.
Still further, it would be advantageous to administer therapy of accelerated arrhythmias more effectively. For example, supraventricular arrhythmias and ventricular arrhythmias may be confused with one another. Each may exhibit similar symptoms such as increased and variable ventricular rates. Supraventricular arrhythmias require therapy to be applied to the atria while ventricular arrhythmias require therapy to be applied to the ventricles. Therapy applied to the incorrect chambers can be potentially dangerous to the patient and result in a waste of stored energy.
Still further, some ventricular arrhythmias may be tolerated by patients without the need of therapy intervention. For example, low rate ventricular tachyarrhythmias may not seriously compromise cardiac output and may even revert back to normal sinus rhythm on their own. Applying electrical therapy to such rhythms would represent needless use of energy and may cause a patient needless discomfort.
Hence, there is a need for effective management of accelerated rhythms. Such management should be capable of discriminating ventricular from supraventricular arrhythmias, discerning the need for therapy, and then applying therapy when the least amount of energy is required for successful treatment.
The present invention provides an implantable cardiac stimulation device and method which effectively manages accelerated arrhythmias. In accordance with the broader aspects of the present invention, electrogram spectral analysis is utilized for arrhythmia discrimination, tolerance discernment, and/or therapy delivery timing.
Arrhythmia discrimination is rendered possible because the physiology of the ventricles is different than the atria. For example, the ventricles are larger than the atria and the ventricular myocardium is thicker than the atrial myocardium. As a result, the distribution of the frequencies of depolarization for the ventricles is different than for the atria. A spectral analysis of an electrogram taken during an arrhythmic episode and having both atrial and ventricular components derived from, for example, a sensing electrode in or near one of the atria and a sensing electrode in or near one of the ventricles may thus be used to discriminate between a ventricular arrhythmia, requiring ventricular therapy and a supraventricular arrhythmia, requiring atrial therapy.
The frequency distribution of depolarization amplitude may also be used to determine if a ventricular arrhythmia is tolerable. This tolerance discernment of a tolerable ventricular tachyarrhythmia results in the withholding of unnecessary and energy consuming therapy.
If the arrhythmia is one requiring therapy, the frequency of maximum depolarization amplitude may be used for therapy delivery timing. When this frequency occurs, the maximum number of myocardial cells will already be depolarized leaving a minimum number of cells left to be depolarized by a defibrillation or cardioverting shock. The shock may be timed by setting a filter to the frequency of maximum depolarization amplitude. When the filter produces an output, the shock may be delivered. In some instances, the shock may be more effective if delayed with respect to the filter output. In either event, the therapy is delivered in timed relation to a depolarization having a frequency equal to the frequency of maximum depolarization.
In accordance with the present invention, an arrhythmia detector initially detects an accelerated arrhythmia of the heart. The initial determination may be based upon ventricular rate, ventricular rate and variability, or atrial rate. A data acquisition system provides an electrogram of the heart preferably including both atrial and ventricular depolarization components in response to the initial arrhythmia detection. The electrogram may be derived from a sensing electrode in the superior vena cava (SVC) and a sensing electrode in the right ventricle (RV) for example. The electrogram is then spectral analyzed to provide spectral data relating to the accelerated arrhythmia. The spectral data preferably includes depolarization amplitudes versus frequency. The spectral data is then used by a pulse generator that applies therapy responsive to the spectral data.
The chambers to receive therapy may be selected by the pulse generator based upon the frequency distribution of the depolarizations. The pulse generator may withhold therapy if the spectral data indicates the presence of a tolerable arrhythmia. Lastly, the pulse generator may time delivery of therapy to a frequency of maximum depolarization amplitude in order to effect a minimum cardioversion or defibrillation threshold.